Why does gamma radiation have a low ionising ability? It is commonly taught that alpha particles have high ionising ability and low penetrating ability (explained by saying due to their large charge and kinetic energy, they are able to knock off electrons of atoms during a collision).
However, what I do not understand is why do gamma radiation does not have a high ionising ability. By $E=hf$ gamma radiation should have quite a lot of energy, so why doesn't it ionise atoms? Even X-Rays in Milikan's oil drop experiment could ionise oil drops, so why can't the more energetic gamma rays?
 A: 
why do gamma radiation does not have a high ionising ability.

This isn't true. Gamma rays have energies of $\gtrsim 100$ keV, which is orders of magnitude higher than the energy required to ionize matter. They are ionizing radiation.
You may be misunderstanding something you've heard about the fact that some gamma rays are highly penetrating. Gamma rays interact with matter via the photoelectric effect, Compton scattering, and pair production. Each of these processes has some probability, which depends on the energy of the gamma, and also, for some of these processes, on the atomic number of the atom. Low-energy gammas, bordering on the x-ray region, are relatively non-penetrating, but higher-energy gammas will typically pass through a fairly large amount of matter without interacting. For example, one might use a lead brick as shielding against high-energy gammas. So in this sense, high-energy gammas may be relatively unlikely to produce ionization in a given object, if the object is not very thick and has a low atomic number.
A: Gammas when they interact with matter have high probability of e+e- pair creation , and those e+e- will ionize for sure.
If their energy is smaller than 1MeV which is the limit for pair creation, they will interact via the Compton effect. There will be energy transferred , if they hit an electron. Alpha particles have the advantage of carrying charge which interacts directly with other charges, but gamma rays do interact.
